Such a wire cord is commonly used for reinforcing elastomer products, such as tires. All or some of the individual wires are crimped before they are twisted together. The crimping of the wires results in a higher elongation at rupture of the cord and warrants a better elastomer penetration into the cord.
There are different prior art techniques for manufacturing high elongation cords that consists of crimped steel wires that are twisted together.
U.S. Pat. No. 5,707,467 discloses crimping the wires in a revolving cam-like pre-former before twisting them together. Such a cam-like pre-former comprises a plate-like or tubular rotary member with 3 to 4 staggered pins. The wire is guided in a zigzag path along the staggered pins and the pre-former is rotated along the wire axis, whereby the pre-former pre-forms a helical wavy form in the wire. Each wire is pre-formed in a separate pre-former. The crimped wires are introduced through a die and a hollow shaft into a rotating buncher type twister, inside which they are twisted together into a cord that is wound on a take-up bobbin. This method has major drawbacks. The zigzag path of the wire in the revolving pre-former requires a limitation of the pulling speed of the wire, and this results of course in a lower productivity. The pre-formers must all be driven in rotation at a controlled speed, which is difficult to achieve. Last but not least, the crimped wires are smoothened again when they are guided over guide rolls and through guide dies before being twisted together.
U.S. Pat. No. 5,111,649 discloses crimping the wires between the meshing teeth of a pair of gear-like wheels. Downstream of the gear-like wheels the crimped wires pass through through-holes in a stationary plate before they are introduced into a twisting machine that twists them together into a steel cord. This method has major drawbacks, too. The toothed wheels can only provide a relatively flat deformation of the wires without risking to damage them. Furthermore, the stationary plate guiding the crimped wires into the twisting machine has a tendency to smooth them again.
Also U.S. Pat. No. 6,311,466 discloses crimping the wires between toothed wheels. However, instead of using only one pair of toothed wheels, one suggests to use a second pair of toothed wheels that is placed next to the first pair in order to pre-form the wire in a plane turned by 90 degrees compared to the first crimping plane and with a different pitch than the first pair. Each wire passes through a separate toothed wheels arrangement. Thereafter, the crimped wires are bundled and introduced into a known twisting machine to be twisted together. According to U.S. Pat. No. 6,311,466, the individual steel wires should thus receive a spatial deformation before they are twisted together, which is said to improve rubber penetration, to increase elongation at rupture and to decrease the stiffness of the cord. It will, however, be appreciated that the wire has a tendency to tilt when it leaves the first pair of toothed wheels. Thus, the second pair of toothed wheels tends to generate the second wave in the same plane as the first wave, which partially ruins the expected advantages. Moreover, this method also suffers from a smoothing back of the crimped wires prior to the final twisting operation.
WO 02/088459 discloses a method for manufacturing a wire cord comprising the steps of bundling a plurality of wires in a bundling means; crimping the wires between meshing toothed surfaces; and twisting together the plurality of crimped wires along a twisting path.